An Fe--B--R based permanent magnet, of which an Fe--B--Nd based permanent magnet is representative, is practically used in various applications, because it is produced from an inexpensive material rich in natural resources and has a high magnetic characteristic, as compared with an Sm--Co based permanent magnet.
However, the Fe--B--R based permanent magnet is liable to be corroded by oxidation in the atmosphere, because it contains highly reactive R and Fe. When the Fe--B--R based permanent magnet is used without being subjected to any surface treatment, the corrosion of the magnet is advanced from its surface due to the presence of a small amount of acid, alkali and/or water to produce rust, thereby bringing about the degradation and dispersion of the magnetic characteristic. Further, when the magnet having the rust produced therein is incorporated into a device such as a magnetic circuit, there is a possibility that the rust is scattered to pollute surrounding parts or components.
There is an already proposed magnet which has a corrosion-resistant metal-plated film on its surface, which is formed by a wet plating process such as an electro-less plating process and an electroplating process, in order to improve the corrosion resistance of the Fe--B--R based permanent magnet with the above-described point in view (see Japanese Patent Publication No.3-74012). In this process, however, an acidic or alkaline solution used in a pretreatment prior to the plating treatment may remain in pores on the magnet, whereby the magnet may be corroded with the passage of time in some cases. In addition, the magnet is poor in resistance to chemicals and for this reason, the surface of the magnet may be corroded during the plating treatment. Further, even if the metal-plated film is formed on the surface of the magnet, as described above, if the magnet is subjected to a corrosion resistance test under conditions of a temperature of 60.degree. C. and a relative humidity of 90%, the magnetic characteristic of the magnet may be degraded by 10% or more from an initial value after lapse of 100 hours.
There is also a conventionally proposed process in which an oxidation-resistant chemical conversion coating film such as a phosphate film or a chromate film is formed on the surface of an Fe--B--R based permanent magnet (see Japanese Patent Publication No.4-22008). The film formed in this process is excellent in adhesion to the magnet, but if it is subjected to a corrosion resistance test under conditions of a temperature of 60.degree. C. and a relative humidity of 90%, the magnetic characteristic of the magnet may be degraded by 10% or more from an initial value after lapse of 300 hours.
A process which has been conventionally proposed in order to improve the corrosion resistance of the Fe--B--R based permanent magnet, and in which a chromate treatment is carried out after the formation of an aluminum film by a vapor deposition process, i.e., a so-called aluminum-chromate treating process (see Japanese Patent Publication No.6-66173), is intended to improve the corrosion resistance of the magnet remarkably. However, the chromate treatment used in this process uses hexa-valent chromium which is undesirable for the environment and for this reason, a waste-liquid treating method is complicated. It is feared that a film formed in this process affects a human body during handling of the magnet, because it contains just a small amount of hexa-valent chromium.
Accordingly, it is an object of the present invention to provide an Fe--B--R based permanent magnet which has, on the surface thereof, a corrosion-resistant film having an excellent adhesion to the magnet, and which can exhibit a stable high magnetic characteristic that cannot deteriorate even if the magnet is left to stand for a long time under high-temperature and high-humidity conditions of a temperature of 80.degree. C. and a relative humidity of 90%, and in which the film is free from hexa-valent chromium, and a process for producing the same.